Overall Summary of Progress: In the previous funding period, we found that DNA damage in cells or tissues drives altered metabolism, activation of NF- B, FOXO3a, and UPRmit, an increase in reactive oxygen species and senescence. This clearly demonstrates cell autonomous mechanisms in response to endogenous DNA damage. This work relied heavily on reporter mouse strains. In addition, using tissue-specific mutant mice, we discovered that in some but not all cases, cells or tissues with increased endogenous DNA damage can drive loss of tissue homeostasis and aging in other non-damaged tissues. Our preliminary work indicates that damaged immune cells are the most potent in driving aging via cell non-autonomous mechanisms. Considerable effort and resources were dedicated to developing these unique transgenic mice that enable us to drill down and discover where, when and how spontaneous time-dependent accumulation of stochastic damage drives aging. During the previous funding period, we developed 85 unique strains of transgenic mice in which the DNA repair complex ERCC1-XPF is depleted or knocked-out systemically or tissue-specifically +/- a variety of reporter constructs to measure senescence markers, autophagy, or stress response activation +/- mutation of key regulators of lifespan/healthspan (e.g., mTOR, catalase, NRF2, NF- B, ATM, p53, p21Cip1 or p16Ink4a).